Most likely its fuel peculation due to todays fuel mixtures. The fuel evaporates very quickly and this is a problem for carburetor equipped cars. I put one of these gaskets on and it has pretty much eliminated the evaporation:

http://store.summitracing.com/partdetail...mp;autoview=sku

I was really surprised how well it worked. I'd also switch it a 180 degree thermostat if you haven't already. And block the heat cross overs on the intake manifold helps too.